The present invention relates to recordable optical disks and optical disk recording and reproducing apparatuses and, in particular, a method for determining a write strategy and a recording power level.
Optical information recording technology, i.e., the technology for recording data onto recordable optical disks has remarkably advanced in recent years. With the advance of the technology, various types of optical disk recording and reproducing apparatuses have been developed. In particular, the applications to storage devices external to computers, such as DVD-RAM drives, have come into widespread use.
Here, optical information recording technology especially refers to the following recording technology. An optical head (which is also referred to as a pickup) radiates the recording layer of a recordable optical disk with laser light. The portions of the recording layer radiated with the laser light change the properties, thereby changing the optical reflectances (which are hereafter referred to as reflectances). Accordingly, changes in radiation power of the laser light between two levels essentially achieve changes in reflectance of the recording layer between two values from area to area. Portions with the low and high reflectances in the recording layer are referred to as recording marks and recording spaces, respectively. According to the optical information recording technology, data is recorded as a sequence of recording marks and recording spaces on the recordable optical disk. For example, in mark-edge recording scheme, each boundary between the recording marks and the recording spaces (namely a mark edge) represents a pulse edge of a digital signal, while each length of the recording marks represents a pulse width of the digital signal.
Recordable optical disks are classified as write-once type or rewritable type.
A write-once optical disk refers to an optical disk onto which data can be recorded only once. The write-once optical disks include CD-Rs (Recordable) and DVD-Rs.
In the write-once optical disks, recording marks are formed as follows. A recording layer contains organic dyes. When the recording layer is radiated with laser light of a predetermined power, the organic dyes are decomposed and thereby the reflectance decreases. Thus, portions of the recording layer radiated with the laser light form recording marks.
The write-once optical disks allow data recording only once for the following reason. The formation of the recording marks generates large amounts of heat at the portions of the recording layer radiated with the laser light. The heat deforms the likes of plastic portions of the surroundings. Since the deformation is irreversible, the portions cannot be restored to the original state before radiated with the laser light. Therefore, the write-once optical disks allow data recording only once.
A rewritable optical disk refers to an optical disk on which data can be recorded and erased multiple times. The rewritable optical disks include CD-RWs (ReWritable), DVD-RAMs, DVD-RWs, DVD+RWs, and so on.
In phase-change types of the rewritable optical disks, recording marks are formed as follows. A recording layer contains alloys, which have two types of solid phases, a crystal phase and an amorphous phase. The reflectance of the recording layer is high in the crystal phase and low in the amorphous phase. Accordingly, the portions in the amorphous phase of the recording layer form recording marks. The formation of the recording marks, i.e., the transition from the crystal phase to the amorphous phase is achieved as follows. The recording layer is radiated with pulses of laser light of a rather high power level. Thereby, small areas of the recording layer are instantaneously heated to a temperature equal to or exceeding the melting point, and then, rapidly cooled to a temperature equal to or below the glass transition point. As a result, the small areas of the recording layer undergo the transition to the amorphous phase.
The phase-change types of the rewritable optical disks allow existing recording marks to be erased as follows. The recording marks are portions in the amorphous phase of the recording layer as described above. Accordingly, a transition from the amorphous phase to the crystal phase within the areas of the recording marks may erase the recording marks.
The deletion of the recording marks, i.e., the transition from the amorphous phase to the crystal phase is achieved as follows. The recording layer of the rewritable optical disk during the revolution is radiated for a relatively long time with laser light of a relatively low power level. Thereby, large areas of the recording layer are once heated to a temperature beyond the glass transition point and below the melting point, and then, cooled with a relatively slow pace. As a result, the large areas of the recording layer undergo the transition to the crystal phase.
Actual data recordings onto the phase-change types of the rewritable optical disks use the laser light emitted with changes in power between the above-mentioned high and low levels. Thereby, the deletions and formations of the recording marks are alternately performed, and an overwriting of data can be achieved on the optical disk.
A known apparatus for recording and reproducing data onto and from the above-mentioned recordable optical disks is, for example, an optical disk recording and reproducing apparatus disclosed in Japanese Laid-Open Patent Publication No. 2000-200418.
FIG. 23 is a block diagram showing an example of the prior art optical disk recording and reproducing apparatus.
Regarding the prior art optical disk recording and reproducing apparatus, the following describes an example of its reproducing block.
A spindle motor 14 revolves an optical disk D around its center axis.
In a data reproducing, a pickup 1 radiates the optical disk D with laser light and converts the reflected light into an analog signal as follows. A semiconductor laser 1a emits laser light at a predetermined power level. The power level is referred to as a reproducing power level. Since the reproducing power level is sufficiently low, the laser light causes no change in the property of the recording layer of the optical disk D. The laser light R1 emitted by the semiconductor laser 1a passes through a collimator lens 1b, a splitter 1c, and an objective lens 1d in this order, thereby focused and reflected within the optical disk D. The reflected laser light R2 passes through the objective lens 1d, the splitter 1c, and a detection lens 1e in this order, thereby focused onto a photodetector 1f. The photodetector 1f detects and converts the reflected laser light R2 into an analog signal d1. Then, the amplitude of the analog signal d1 is substantially proportional to the intensity of the reflected laser light R2.
A head amplifier 2 receives and amplifies the analog signal d1 from the pickup 1. An equalizer 3 performs shaping on the analog signal d2 amplified by the head amplifier 2. A binarizer 4 compares the analog signal d3 undergoing the shaping of the equalizer 3 with a predetermined threshold value, and binarizes the analog signal d3 with respect to the threshold value, thereby converting the analog signal d3 undergoing the shaping into a digital signal d4. A phase locked loop (PLL) 5 synchronizes the digital signal d4 and a reference clock signal d5a, and then a digital signal d5 is decoded into data.
Regarding the prior art optical disk recording and reproducing apparatus, the following describes an example of its recording block.
A recording-pattern-determination section 8 determines a recording pattern corresponding to target data of recording. The recording pattern is a rectangular pulse train representing a sequence of recording marks and recording spaces to be written onto the optical disk D. In the recording pattern, a pulse width, i.e., an assertion time, represents the length of one of the recording marks (which is hereafter referred to as a mark length). In addition, an interval from the rear end of a pulse to the front end of the next pulse (which is hereafter referred to as a negation time) represents the length of one of the recording spaces (which is hereafter referred to as a space length).
A recording-pulse-determination section 9 determines recording pulses d9 based on the recording pattern d8 determined by the recording-pattern-determination section 8. Here, the recording pulses d9 are rectangular pulses, and the pulse widths and intervals represent the pulse widths and intervals of the laser pulses that the semiconductor laser 1a should emit. The recording pattern d8 is converted into the recording pulses d9 according to fixed conversion criteria. In particular, one pulse in the recording pattern d8 is generally divided into more than one of the recording pulses d9. Then, the pulse width of the recording pulses d9 is generally narrower than the pulse width of the recording pattern d8. Thereby, an actual value of the mark length does not exceed the width of the corresponding pulse in the recording pattern d8, when heat resulting from the laser light is diffused outside the radiated area. A set of the above-mentioned conversion criteria is referred to as a write strategy, or alternatively, a recording pulse condition or a recording pulse structure. The detail of the write strategy is described later.
A recording-power-determination section 12, at the time of a data recording, determines laser pulse heights of the semiconductor laser 1a, i.e., power levels of the semiconductor laser 1a at a constant level. The power level determined is referred to as a recording power level. The recording power level d12 is sent out to a laser driver 13 along with the recording pulses d9.
The laser driver 13 controls a driving current d13 in the semiconductor laser 1a according to the recording pulses d9 and the recording power level d12, and, especially during assertion times of the recording pulses d9, conducts an amount of the driving current d13 corresponding to the recording power level d12. Thereby, the semiconductor laser 1a emits laser pulses substantially identical in shape with the recording pulses d9 at the recording power level d12.
The actual shapes of the recording marks are not uniquely determined only by the recording pulses and the recording power levels. For example, fluctuations of an ambient temperature causes fluctuations of the areas changing into the amorphous phase in the recording layer of the optical disk D, since the cooling rate of the recording layer depends on the ambient temperature. Furthermore, the wavelength of laser light of a semiconductor laser fluctuates in a range substantially proportional to the range of fluctuations of temperature, and varies around a standard value from one device to another. In the case of DVD-Rs, for example, the fluctuations of wavelength of the laser light change the amounts of energy absorbed by the recording layer, since light absorption characteristics of the organic dye depend on the wavelength of the light absorbed. In addition, the likes of the structures of optical disks vary from one product to another. Such variables cause distortions of the recording marks. Accordingly, the formation of the recording marks, especially the positioning of the mark edges is performed with an insufficient accuracy when the recording pulses and the recording power levels are determined merely according to standards of write strategy and recording power condition. This causes high error rates of data actually recorded.
The prior art optical disk recording and reproducing apparatuses corrects the write strategy and calibrates the recording power levels in a manner appropriate to the optical disks and the apparatuses, in order to improve the accuracy of formation of the recording marks. The following describes an example of a write-strategy-correction block and a recording-power-calibration block in the prior art optical disk recording and reproducing apparatus.
A β-value measurement section 11 measures the β value of the analog signal d3 undergoing the shaping of the equalizer 3. The β value of an analog signal is defined as the formula: β=(a+b)/(a−b), using the maximum level a and the minimum level b within one period of the analog signal. The β value of the analog signal is equal to the mesial level of the waveform ((a+b)/2) normalized with the amplitude (a−b).
The β value of the analog signal is a parameter used as one of the determinants of the recording power levels of the semiconductor laser 1a as follows. The binarizer 4 binarizes the analog signal d1 reproduced by the pickup 1 with respect to a predetermined threshold value. The error rate of digital data rises in the binarization, when the mesial level of the waveform of the analog signal d1 largely shifts from the threshold value and then the β value largely shifts from a target value. Accordingly, reduction of the error rate equal to or below a predetermined, acceptable limit requires the β value of the analog signal d1 within acceptable limits. Since the reflectances and shapes of the recording marks substantially determine the β value of the analog signal d1, the recording power levels of the semiconductor laser 1a determines the β value. Conversely, a fixed β value of the analog signal d1 determines corresponding levels of the recording power. A recording power condition refers to correspondences between parameters representing the quality of the analog signal and the recording power levels, such as the correspondence between the β value of the analog signal d1 and the recording power levels.
On an optical disk D, histories of the write strategies and the recording power conditions used in the previous data recordings are stored, together with a standard write strategy and a standard recording power condition defined by specifications. A write strategy decoder 60 decodes a digital signal d5 received from the PLL 5 into a write strategy d6 and provides it to a write-strategy-correction section 7. A recording-power-condition decoder 100 decodes the digital signal d5 received from the PLL 5 into a recording power condition d10 and provides it to a recording-power-determination section 12.
An edge shift detector 20 receives a digital signal d4 from the binarizer 4, and a clock signal d5a from the PLL 5. Then, the edge shift detector 20 detects front-end edge shifts d20a and rear-end edge shifts d20b on the digital signal d4, through a comparison between the digital signal d4 and the clock signal d5a. Here, the edge shifts refer to phase differences between the digital signal d4 and the clock signal d5a, which is expressed in terms of time. The detected edge shifts d20a and d20b are provided to the write-strategy-correction section 7.
The write-strategy-correction section 7 receives the write strategy d6 from the write strategy decoder 60 and stores it in an inside memory. In a correction to the write strategy d6 stored, the write-strategy-correction section 7 compares the front-end edge shifts d20a and the rear-end edge shifts d20b of the digital signal d4 with predetermined, acceptable limits. The section brings results of the comparisons into correspondence with the write strategy d6 stored and stores the results, then correcting the write strategy d6 stored with predetermined correction values. The section stores the write strategy d7 corrected and provides it for the recording-pulse-determination section 9.
The following describes a relation between a recording pattern and recording pulses. FIG. 7 is a schematic diagram showing a relation between a recording pattern and recording pulses when a DVD-R is used as the optical disk D. The parts (a)-(c) of FIG. 7 show waveforms of a recording pattern, recording pulses, and laser pulses of the semiconductor laser 1a, respectively. The part (d) of FIG. 7 shows recording marks M and recording spaces S formed in the recording layer of the optical disk D by the laser pulses shown in (c) of FIG. 7. Here, let T denotes the unit of the pulse widths. The unit length 1 T corresponds to a clock period. Each of the pulse widths and intervals in the recording pattern is set substantially equal to an integral multiple of the clock period. Furthermore, a recording speed of FIG. 7 equals a standard reproducing speed (namely single speed).
When the recording layer of the DVD-R is radiated with laser light of the semiconductor laser 1a, heat resulting from the laser light is diffused from the radiated area into the surroundings. The heat propagates especially along a groove of the DVD-R. Accordingly, laser pulses substantially identical in shape with the recording pattern actually form the recording marks longer than the corresponding portions of the recording pattern. When the heat further reaches the preceding or subsequent recording mark through the recording spaces, the preceding or subsequent recording mark is distorted. In particular, the mark edges shift from the positions corresponding to the recording pattern. The distortions cause errors in the data recording.
In order to avoid such distortions, the prior art optical disk recording and reproducing apparatus converts the recording pattern into the following recording pulses. In particular, one pulse in the recording pattern is generally divided into two or more of the recording pulses. Then, each width of the recording pulses is narrower than the pulse widths of the recording pattern. Thereby, the amount of heat delivered from the laser light to the recording layer of the DVD-R is reduced in the formation of one of the recording marks.
Each pulse in the recording pattern has a width integral times as long as the clock period T. Here, the minimum pulse width in the recording pattern is triple the clock period T.
A top pulse refers to the first pulse in a train of the recording pulses corresponding to a pulse in the recording pattern. The front end of the top pulse is set at a point corresponding to the front end of the pulse in the recording pattern with a lag of a predetermined length (which is hereafter referred to as a front-end lag). The rear end of the top pulse is set at a point the minimum pulse width 3 T behind the front end of the pulse in the recording pattern. If the pulse in the recording pattern is a pulse of the minimum width, then the rear end of the pulse agrees with the rear end of the top pulse. Thus, the top pulse has a width narrower than the minimum pulse width 3 T in the recording pattern.
Multi-pulses, i.e., a pulse train having a period equal to the clock period T are set during the interval from the rear end of the top pulse to a point corresponding to the rear end of the pulse in the recording pattern. The rear extremity of the multi-pulses agrees with the rear end of the pulse in the recording pattern since each pulse width of the recording pattern is an integral multiple of the clock period T. The multi-pulses have a constant pulse width and a constant pulse interval.
Each pulse in the recording pattern is converted into two or more of the recording pulses generally including a top pulse and multi-pulses as described above. In particular, each pulse width of the top pulse and the multi-pulses is narrower than that of the corresponding pulse in the recording pattern. Thereby, the amount of heat delivered from the laser light to the recording layer of the DVD-R is reduced in the formation of one of the recording marks. This prevents excessively large size of the recording marks from appearing, and excessive amounts of heat from being diffused into the adjacent recording marks.
The front-end lags of the top pulses and the pulse widths of the multi-pulses in the recording pulses are further optimized for proper correspondences between ends of the pulses in the recording pattern and edges of the recording marks as follows.
For example, a recording pattern is assumed as shown in (a) of FIG. 7. The recording pattern consists of a first pulse P1 having a width of 7 T, a pulse separation having a length of 3 T, and a second pulse P2 having a width of 3 T, in this order. Recording pulses shown in (b) of FIG. 7 correspond to the recording pattern.
The portion of the recording pulses corresponding to the first pulse P1 in the recording pattern consists of a first top pulse P10 and multi-pulses P11.
The first top pulse P10 has a width Tt1=p1×T (p1: a positive rational number). The front end P10a of the first top pulse P10 is set at a point corresponding to the front end P1a of the first pulse P1 in the recording pattern with a front-end lag F1=f1×T (f1: a positive rational number). On the other hand, the rear end P10b of the first top pulse P10 is set at a point corresponding to the front end P1a of the first pulse P1 in the recording pattern with a lag of 3 T. Thus, f1+p1=3.
The multi-pulses P11 have a constant period of 1 T. Each pulse of the multi-pulses P11 has a constant width Tm=m×T (m: a positive rational number). The interval from the rear end P10b of the first top pulse P10 to the front extremity P11a of the multi-pulses P11 and negation times in the multi-pulses P11 each have a constant value Sm=s×T (s: a positive rational number). Thus, m+s=1. The rear extremity P11b of the multi-pulses P11 agrees with the rear end P1b of the first pulse P1 in the recording pattern.
The portion corresponding to the second pulse P2 in the recording pattern consists of a second top pulse P20 only. The second top pulse P20 has a width Tt2=p2×T (p2: a positive rational number). The front end P20a of the second top pulse P20 is set at a point corresponding to the front end P2a of the second pulse P2 in the recording pattern with a front-end lag F2=f2×T (f2: a positive rational number). The rear end P20b of the second top pulse P20 agrees with the rear end P2b of the second pulse P2 in the recording pattern. Thus, f2+p2=3.
The semiconductor laser 1a emits laser pulses substantially identical in waveform with the above-mentioned recording pulses. The part (c) of FIG. 7 shows the waveform of the laser pulses. The height H0 of the laser pulses represents the recording power level of the semiconductor laser 1a. The radiation of the laser pulses forms a train of recording marks M and recording spaces S into the recording layer of the optical disk D as shown in (d) of FIG. 7.
When the front-end lag of the top pulse and the pulse width of the multi-pulses in the recording pulses are optimum, the train of recording marks and recording spaces properly corresponds to the recording pattern as shown in (a) and (d) of FIG. 7. More specifically, the front-end lags f1 and f2 and the pulse width m of the multi-pulses P11, which are expressed in the unit of the clock period T, are fixed at respective optimum values selected among predetermined values as follows.
The optimum pulse widths of the multi-pulses are set in advance for respective widths of the corresponding pulses in the recording pattern (namely mark lengths). The set values are determined mainly for bringing the rear edges of the recording marks into good agreement with the rear ends of the pulses in the recording pattern. For example, a value corresponding to the mark length 7 T is selected among the set values as the pulse width m of the multi-pulses P11.
The optimum values of the front-end lags are set in advance for respective combinations of widths of the corresponding pulse in the recording pattern (namely mark lengths) and intervals between the front end of the pulse and the rear end of the preceding pulse (which are hereafter referred to as preceding space lengths), as shown in the following Table 1. The set values are determined mainly for bringing the front edges of the recording marks into good agreement with the front ends of the pulses in the recording pattern.
Table 1 is a list showing correspondences between the front-end lags Fij (i, j=3, . . . , 5) and combinations of the mark lengths and the preceding space lengths.
TABLE 1MARK LENGTH≧5T4T3TPRECEDING≧5TF55F54F53SPACE  4TF45F44F43LENGTHS  3TF35F34F33
Here, each value of the front-end lags Fij (i, j=3, . . . , 5) is expressed by a rational number in the unit of the clock period T. In FIG. 7, for example, the mark length and the preceding space length of the second pulse P2 in the recording pattern are both equal to 3 T. Then, the front-end lag F2 of the second top pulse P20 in the recording pulses is set at the value F33 corresponding to the combination of the mark length 3 T and the preceding space length 3 T according to Table 1.
In this description of the invention, the write strategy refers to the above-mentioned criteria on which to determine waveforms of the recording pulses corresponding to the recording pattern, especially positions of the ends of the recording pulses, based on the mark lengths and the space lengths in the recording pattern. For example, the write strategies for DVD-Rs and DVD-RWs are criteria on which to determine: (a) correspondences between the pulse widths of the multi-pulses of the recording pulses and the mark lengths in the recording pattern; and (b) correspondences between the front-end lags of the top pulses of the recording pulses and the combinations of the mark lengths and the preceding space lengths in the recording pattern as shown in Table 1. On the other hand, the write strategy for DVD-RAMs includes, in addition to the above-mentioned criteria for the front-end lags of the top pulses, another criteria on which to determine an amount by which the rear extremity of the multi-pulses or the rear end of the last pulse following the multi-pulses leads the rear end of the corresponding pulse in the recording pattern (namely rear-end leads).
The prior art optical disk recording and reproducing apparatus as shown in FIG. 23, at the beginning of a data recording, optimizes the write strategy and the recording power condition for the recording-pulse-determination section 9 and the recording-power-determination section 12, respectively, as follows.
On the optical disk D, the standard write strategy and the standard recording power condition are stored in advance, and, in addition, the histories of write strategy and recording power condition used in the previous data recordings are stored. The prior art optical disk recording and reproducing apparatus first selects one write strategy and one recording power condition from among the ones stored on the optical disk D, and reads the selections as the initial conditions. The reading is similar to the ordinary data reproduction. The pickup 1 reproduces an analog signal d1 from the optical disk D. The analog signal d1 is converted into a digital signal d5 through the head amplifier 2, the equalizer 3, the binarizer 4, and the PLL 5 (see FIG. 23). The write strategy decoder 60 and the recording-power-condition decoder 100 decode the digital signal d5 into the write strategy and the recording power condition of the initial conditions, respectively.
The decoded write strategy d6 is provided for and stored in the write-strategy-correction section 7. The decoded write strategy d6 further passes through the write-strategy-correction section 7 and enters the recording-pulse-determination section 9. On the other hand, the decoded recording power condition d10 is provided for the recording-power-determination section 12.
For neither the optical disk nor the optical disk recording and reproducing apparatus, the write strategy selected as the initial conditions is generally optimum. Accordingly, the write strategy is corrected as follows. First, the recording-pattern-determination section 8 outputs a predetermined test recording pattern d8. The recording-pulse-determination section 9 determines test recording pulses d9 corresponding to the test recording pattern d8 according to the write strategy selected as the initial conditions. The recording-power-determination section 12 determines recording power levels d12 according to the recording power condition selected as the initial conditions. The laser driver 13 drives and causes the semiconductor laser 1a to emit laser light R1 at the recording power level d12. Thereby, a train of recording marks corresponding to the test recording pattern (namely test recording marks) is formed in a power calibration area (PCA) on the optical disk D. The pickup 1 radiates the test recording marks in the PCA with laser light of the reproducing power level and detects the reflected light. Variations in intensity of the reflected light are transmitted as an analog signal d1 and converted into a digital signal d4 through the head amplifier 2, the equalizer 3, and the binarizer 4. The PLL 5 synchronizes the digital signal d4 to the clock signal d5a, and in addition, provides the clock signal d5a for the edge shift detector 20. The edge shift detector 20 compares the digital signal d4 from the binarizer 4 with the clock signal d5a from the PLL 5, thereby detecting the front-end edge shifts d20a and the rear-end edge shifts d20b on the digital signal d4. The write-strategy-correction section 7 compares the front-end edge shifts d20a and the rear-end edge shifts d20b with respective, acceptable limits. The section brings results of the comparisons into correspondence with the current write strategy d6 and stores the results. The write-strategy-correction section 7 further corrects the write strategy d6 with predetermined correction values and provides it as a new write strategy d7 for the recording-pulse-determination section 9. The recording-pulse-determination section 9 determines new test recording pulses d9 based on the test recording pattern d8 according to the new write strategy d7. The above-described process is repeated for write strategies with various correction values. Among the write strategies, one write strategy is selected, which achieves both the front-end edge shifts d20a and the rear-end edge shifts d20b at or below the acceptable limits. Thus, the write strategy is optimized.
After the optimization of the write strategy, a calibration of recording power level is performed as follows. The recording-pattern-determination section 8 outputs another test recording pattern d8. The recording-pulse-determination section 9 determines test recording pulses d9 based on the test recording pattern d8. The recording-power-determination section 12 sets the recording power level d12 at a predetermined initial value. As the initial value, a recording power level corresponding to a target β value is selected from the recording power condition. Here, the target β values are, for example, set in advance for the optical disk recording and reproducing apparatus to be appropriate to respective types of the recordable optical disks. The setting reduces error rates of the digital signal reproduced at or below predetermined acceptable limits. The laser driver 13 drives and causes the semiconductor laser 1a to emit laser light R1 at the recording power level d12. Thereby, test recording marks are formed in the PCA on the optical disk D.
The pickup 1 radiates the test recording marks in the PCA with laser light of the reproducing power level, and detects the reflected light. Variations in intensity of the reflected light are transmitted as an analog signal d1. The β-value measurement section 11 measures the β value of the analog signal d1. The β value d11 is stored in the recording-power-determination section 12. Then, the above-described process is repeated for every change of the recording power level from the initial value in predetermined steps. In other words, the β values of the analog signals reproduced from the test recording marks are measured and stored at every time when the recording power level is changed and new recording marks are formed at the recording power level. This produces a list showing correspondences between the numbers of changes in the recording power levels (namely step numbers) and the β values, namely a new recording power condition. The recording power level corresponding to the target β value is selected under the new recording power condition. Thus, the recording power level is optimized. The above-described optimization of the recording power levels is referred to as an optimum power calibration (OPC).
Speeding up the data recording is desired for the above-mentioned optical disk recording and reproducing apparatus. The desire requires an increase in rotation speed of the optical disk during the data recording (namely recording speed). However, when the recording speed is positive-integer n times as high as the single speed (namely n-times speed), the recording marks have been distorted in contrast to that of single-speed recordings. The following experiment and the consideration based on its results has clarified the distortions of the recording marks resulting from high-speed recordings.
FIG. 24 is a schematic diagram showing a recording pattern, recording pulses, and recording marks when the write strategy used in single-speed recordings is adopted into a double-speed recording. The parts (a)-(c) of FIG. 24 show the waveforms of the recording pattern, the recording pulses, and laser pulses of the semiconductor laser 1a, respectively. The part (d) of FIG. 24 shows recording marks M1 and recording spaces S1 formed in the recording layer of the optical disk D by the laser pulses shown in (c) of FIG. 24.
As seen from the comparison between FIGS. 7 and 24, the recording patterns are substantially identical in shape and the recording pulses are substantially identical in shape. Here, being substantially identical in shape refers to having pulse widths and pulse intervals in common, expressed in the unit of the clock period. In FIG. 24, the same reference symbols as those in FIG. 7 designate parts in common with FIG. 7. The recording patterns in (a) of FIG. 7 and (a) of FIG. 24 are substantially identical in shape, and the recording pulses in (b) of FIG. 7 and (b) of FIG. 24 are substantially identical in shape. In (b) of FIG. 24, for example, the first top pulse P10 in the recording pulses has a width Tt1=p1×T1 and a front-end lag F1=f1×T1. The multi-pulses P11 have a pulse width Tm=m×T1 and a negation time Sm=s×T1. The second top pulse P20 has a similar size.
The unit length of pulse width, namely the clock period 1 T1 in FIG. 24 corresponds to ½ of the clock period 1 T in FIG. 7 (1 T1=(½)T). Accordingly, the actual pulse widths and intervals of the recording pattern and the recording pulses in FIG. 24 are half the lengths of those in FIG. 7. On the other hand, the rotation speed of optical disk in FIG. 24 is twice as high as that in FIG. 7. Thus, if the laser pulses are identical in shape with the recording pulses regardless of the rotation speeds, the radiated areas have the same shapes in common regardless of the recording speeds. Then, the recording marks of the same shapes are expected in FIGS. 7 and 24, where a recording power level H1 of FIG. 24 higher than the recording power level H0 of FIG. 7 is required for substantial equalization of absorbed energy densities of the recording marks between single-speed recordings and double-speed recordings.
As seen from the comparison between (d) of FIG. 7 and (d) of FIG. 24, the recording mark M1 resulting from the double-speed recordings is distorted with a tapered front edge Ma in contrast to the recording mark M resulting from the single-speed recordings. The following is considered as the reason for the distortion of the recording mark with the tapered front edge associated with increases in recording speed. The increase in recording speed reduces the pulse widths of the laser light and increases the pulse heights. Accordingly, lags of rising edges of the laser pulses are increased too largely with increase in recording speed to be ignored when compared with the total of the pulse width. As a result, power levels of the laser light are low in the front ends of the pulses, and thereby cause front edges of the recording marks to taper. In the tapered front edge, the mark edge shifts from the front end of the corresponding pulse in the recording pattern. Thus, the edge shifts of the digital signal are increased at the front mark edges. This increases error rates of data.
Further increases in recording speed cause additional distortions of the recording marks as follows. FIG. 25 is a schematic diagram showing recording marks M2 when the write strategy used in single-speed recordings is adopted into a quadruple-speed recording. As seen from the comparison between (d) of FIG. 7 and FIG. 25, the middle portion Mb of the recording mark M2 formed in the quadruple-speed recording is thicker than that of the recording mark M formed in the single-speed recording (shown by broken lines in FIG. 25). In particular, the middle portion Mb exceeds the groove g in width.
Furthermore, the rear edge Mc of the recording mark M2 formed in the quadruple-speed recording is longer than that of the recording mark M formed in the single-speed recording. The distortions reveal that the range from the middle portion Mb to the rear edge Mc of the recording mark M2 is overheated in the quadruple-speed recording.
The following is considered as the reason for the overheated range from the middle portion to the rear edge of the recording mark with increases in recording speed. The increase in recording speed reduces intervals of the laser pulses. Accordingly, the radiation of one of the laser pulses starts immediately after the radiation of the previous one of the laser pulses is finished and before the previously radiated areas cool sufficiently. In particular, the multi-pulses with short pulse intervals cause excessive amounts of heat to be built up in the recording layer. As a result, the range from the middle portion to the rear edge of the recording mark expands beyond a predetermined area.
When the recording mark has a long rear edge, the mark edge shifts from the rear end of the corresponding pulse in the recording pattern. Thus, the edge shifts of the digital signal increase at the rear mark edges. In addition, the above-mentioned excessive amount of heat, when propagating through the recording space to the subsequent recording mark, distorts the front edge of the recording mark, thereby causing the edge shifts of the digital signal to be increased at the front mark edges. The above-described increases of the edge shifts increase error rates of data.
When dusts, for example, settle on the surface of the optical disk D, raising the recording power level is required in order to suppress the increases of error rates of data. However, even ordinary level of the recording power causes excessive amounts of heat to be built up in the latter half portion of the recording mark M2. Accordingly, the increase in recording power level largely distorts the recording marks, thereby increasing the edge shifts and error rates of data, contrary to expectations. In other words, recording power margins substantially decrease with increases in recording speed. Here, the recording power margin refers to the range of the recording power level where the edge shifts may fall within acceptable limits. Thus, the prior art optical disk recording and reproducing apparatus substantially reduces the recording power margins with the increases in recording speed, thereby reducing its reliability of the data recordings.
The following problem, in addition, results from the expansion of the middle portion Mb of the recording mark M2 beyond the width of the groove g as shown in FIG. 25. The grooves g of CD-Rs and DVD-Rs slightly meander and thereby provide predetermined wobble signals. Furthermore, DVD-RWs have land pri-pits (LPPs) L on the land tracks, on which predetermined LPP signals are recorded. The wobble signals and the LPP signals specify, for example, addresses on the groove tracks. When the middle portion Mb of the recording mark M2 excessively expands, the edges of the groove g undergo plastic deformations and, in addition, the reflectances of the LPPs L drop. This reduces the S/N ratios of the wobble signal and the LPP signal, thereby increasing errors of reading addresses.
Avoiding the above-described problems associated with increases in recording speed requires the prior art optical disk recording and reproducing apparatus to change the write strategy and the recording power condition in a manner complicated and appropriate to the recording speed, and adjust the proper shapes of the recording marks. For example, achieving the ability of recording on a common type of optical disks at single- to quadruple-recording speeds requires properly using four types of the write strategies and the recording power conditions for the respective recording speeds.
However, the changes of the write strategies and the recording power conditions depending on the recording speeds cause the following problem. When the write strategy and the recording power condition stored on the prior art optical disk are set as the initial conditions, the corresponding recording speed is generally different from the recording speed of a new data recording. Accordingly, the write strategy and the recording power condition which are set as the initial conditions are not optimum, in general, for the new data recording. Furthermore, no information on the recording speed corresponding to the above-mentioned write strategy and recording power condition is store on the prior art optical disks. Accordingly, the recording speed corresponding to the write strategy and the recording power condition set as the initial conditions are quite different from the recording speed of the new data recording in many cases. Thereby, reductions of error rates of data are difficult through the above-described correction of write strategy and OPC. For example, the write strategy of single-speed recordings is set as the initial conditions for a quadruple-speed recording. In this case, the recording power level is insufficient for the top pulses, but excessive for the multi-pulses. Since the difference between the insufficiency and the excess is generally large, the optimization is difficult by means of the above-described correction of write strategy. Even if the correction may reduce error rates of data below acceptable limits, a large number of parameters is to be adjusted. Therefore, the adjusting time excessively increases the time for the correction of write strategy, thereby delaying the start of the data recording onto the optical disk.